1. Field of the Invention
The present invention relates to tail rotors for helicopters and other rotary wing aircraft. In particular, the present invention relates to a multi-bladed tail rotors and their ability to accommodate potentially powerful Coriolis torque.
2. Description of Related Art
One of the significant challenges involved with the design of multi-bladed tail rotors is their ability to accommodate potentially powerful Coriolis torque. When the rotor plane of a helicopter rotor is tilted relative to the shaft, 1/rev and 2/rev Coriolis torque is generated. Because the 1/rev Coriolis torque is proportional to the coning angle, it is usually negligible for most tail rotors. For two-bladed tail rotors, the 2/rev Coriolis is also not a problem because both blades speed up and slow down at the same time, and the drive system is usually sufficiently flexible to provide the necessary torsional freedom. However, the 2/rev Coriolis torque becomes a problem with multi-bladed tail rotors when no lead-lag articulation is provided.
Various methods are used on existing helicopters with multi-bladed tail rotors to provide the necessary relief for 2/rev Coriolis torque. For example: the Sikorsky S-56 uses a fully articulated rotor having lead-lag hinges and dampers; the Sikorsky S-61 has a flexible spindle at the blade root combined with restricted flapping motion to limit stresses due to Coriolis; the Kaman UH-2 allows a small amount of lead-lag motion by using a rocking pin arrangement in its flapping hinge; and the Lockheed AH-56 uses a gimbaled tail rotor hub that relieves the 2/rev Coriolis torque in the same manner as a two-bladed teetering rotor. Unfortunately, all of these approaches tend to be heavy and complex. They each require highly loaded bearings oscillating at tail rotor frequency. This results in a design that requires a lot of maintenance and a significant amount of downtime.
One of the ways to approach this problem is to mount two, two-bladed rotors on the same shaft. This arrangement provides a four-bladed tail rotor with the mechanical and structural simplicity of a two-bladed teetering rotor. By using this concept, no bearings are required to oscillate while carrying the full centrifugal force of the blade.
The AH-1Z/UH-1Y tail rotor also utilizes this approach, where two 2-bladed rotors are mounted on the same drive shaft. Each assembly is a two-bladed teetering rotor; they are independently mounted on a single output shaft. The span wise axes of the blade-pairs are perpendicular to each other, and are separated axially to provide adequate space for accommodating hub attachment hardware and operational clearance between them. However, this configuration does not inherently provide relief for the 2/rev Coriolis torque. Whenever the tail rotor experiences first harmonic flapping, one pair of blades is trying to speed up at the same instant in time that the other pair of blades is trying to slow down. Thus, the two rotors are trying to move like a pair of scissors.
This approach has been used on several research and production models throughout the rotorcraft industry. Bell Helicopter Textron Inc. has successfully flown a double-teetering tail rotor with coaxial shafts on one of its research aircraft. The AH-64D Apache uses a double-teetering tail rotor with flexible forks. While both these approaches provide the desired relief for 2/rev Coriolis torque, there are several disadvantages associated with each one: the mechanical complexity, heavier design, problems associated with tailoring stiffness of critical metal partsxe2x80x94possibly resulting in a degraded structural design and potentially catastrophic failure modesxe2x80x94just to name a few.
Although the foregoing approaches represent significant strides in the area of tail rotor design, significant challenges remain with regard to the ability of multi-bladed tail rotors to accommodate this potentially powerful Coriolis torque.
While various multi-bladed tail rotor designs presently in use compensate for Coriolis torque differently, the tail rotor system of the present invention offers a simpler and more cost-effective solution by making use of existing parts that are required to perform other functions.
There is a need for a multi-bladed tail rotor system that can accommodate potentially powerful Coriolis torque without the need for heavy, complex components, such as highly loaded bearings oscillating at tail rotor frequencies.
Therefore, it is an object of the present invention to provide a multi-bladed tail rotor system that can accommodate 2/rev Coriolis torque without the need for heavy, complex components that require significant maintenance and downtime.
This object is achieved by providing a four-bladed tail rotor system in which 2/rev Coriolis relief is provided by optimizing the dynamic characteristics of an existing component in the system, i.e., an elastomeric bearing that accommodates rotor flapping. The tail rotor system of the present invention utilizes two stacked two-bladed teetering rotors, each rotor pair being mounted onto the same single drive shaft through a unique rotor yoke assembly. The span wise axes of the two pairs of blades are perpendicular to each other and are separated axially to provide adequate space for accommodating hub attachment hardware and operational clearance. Each rotor yoke assembly is mounted to the drive shaft with a bearing and trunnion assembly in which a pair of trunnion arms having a generally conical shape extend radially outward from a cylindrical body portion.
The trunnion arms are preferably shaped to fit securely within an elastomeric bearing. The elastomeric bearings may be either molded to the trunnion arms or pre-molded and secured to the trunnion arms after molding. A rigid sleeve is disposed around each elastomeric bearing. These sleeves are configured to fit securely within a transverse bore that passes through each rotor yoke. The elastomeric bearings and sleeves are held in place within the yoke by retention fittings that are coupled to the rotor yokes at each end of the transverse bore. The sleeves may include stop members that are received by the retention fittings to limit the movement of the yoke relative to the drive shaft.
In the preferred embodiment of the present invention, an inboard bearing and trunnion assembly, a hub adapter, and an outboard bearing and trunnion assembly are coupled together on the drive shaft by an inboard cone, an outboard cone, and a mast nut. Drive torque is transferred from the drive shaft to the inboard bearing and trunnion assembly through splines on the exterior of the drive shaft which mate with splines on the interior of the body portion of the inboard bearing and trunnion assembly. The drive torque is transferred from the inboard bearing and trunnion assembly to the hub adapter through a toothed coupling on one end of the hub adapter, and from the hub adapter to the outboard bearing and trunnion assembly through another toothed coupling on the other end of the hub adapter.
The multi-bladed tail rotor system according to the present invention provides the significant advantages. Conventional teetering rotors that use elastomeric bearings to provide flapping degrees of freedom, require that the radial stiffness of the bearings to be very high to minimize radial deflection under rotor torque. However, in the multi-bladed tail rotor system according to the present invention, the radial stiffness of a uniquely designed elastomeric flapping bearing is tailored to provide adequate stiffness to react to rotor torque and to provide adequate softness to relieve the 2/rev Coriolis torque, without adding additional hardware. Because this Coriolis relief is provided by tailoring the spring rate of an existing component, the resulting hub assembly provides a much simpler configuration with reduced weight and cost, and higher reliability due to reduction in the number of parts in the system.